1. Technical Field
The present invention relates to a magnetic sensor or the like.
2. Related Art
In the related art, a biomagnetism detecting apparatus has been known that measures a minute magnetic field occurring from a living subject, such as cardiac magnetism (magnetism coming from a heart), cerebral magnetism (magnetism coming from a brain), and the like. As such a biomagnetism detecting apparatus, for example, there is a Superconducting QUantum Interference Device (SQUID). Furthermore, the SQUID is a device (magnetic sensor) that can take variations of a slight magnetic field out as an electric voltage under a low temperature environment, by using, for example, a device (Josephson device) in which a thin portion (Josephson junction) is provided in a part of a superconducting device such as a superconducting ring, or the like.
FIGS. 5A and 5B are schematic diagrams of magnetic flux detecting coils showing an example of the SQUID in the related art. FIG. 5A is a diagram showing a magnetic flux detecting coil (magnetometer), which is wound once. FIG. 5B is a diagram showing a magnetic flux detecting coil that two parallel coils wound in opposite directions to each other are connected in series (first-order gradient type gradiometer).
As shown in FIG. 5A, in a magnetometer 101, a magnetic field 110 coming into the coil is totally detected. Therefore, in order to detect only a magnetic field (for example, cardiac magnetism or cerebral magnetism) generated from near the coil, it is necessary to prepare a separate method of completely eliminating a noise by a magnetic field having the source distant from the coil (for example, external magnetic noise).
As shown in FIG. 5B, in a first-order gradient type gradiometer 102, the magnetic field 110 is detected as a difference of detection signals detected from the two coils wound in the opposite directions to each other. For this reason, the influence of the magnetic field having the source distant from the coils is negated and becomes zero between the two coils, and only the magnetic field generated from near the coils is detected. However, the SQUID costs are high because a superconducting device or a Josephson device is used. In addition, the SQUID needs an effort because it is necessary to frequently supply liquid helium or liquid nitrogen to a cooling system in order to maintain a low temperature environment.
On the other hand, there is an optically-pumped atomic magnetometer as a method of measuring a micro-magnetic field without using the SQUID. The optically-pumped atomic magnetometer is an apparatus that measures a magnetic field by detecting a magnetization state of an atom by causing an atom and a magnetic field to interact with each other using an optical pumping method (a method in which an electron spin of atoms is polarized using polarized light and the polarized atoms are detected with high sensitivity). For example, in Appl. Phys. B75, 605-612 (2002) and Appl. Phys. B76, 325-328 (2003), two laser beams having polarization directions different from each other are incident on a gas cell into which alkali metal atoms such as cesium and the like are injected, the two laser beams transmitted through the gas cell are each received with two photodetectors to detect light intensities. After that, optical signals detected by the two photodetectors are converted into electric signals to calculate a difference in intensity variations of the laser beams, and thereby measuring a micro-magnetic field excluding an influence of an external magnetic field.
However, in the Appl. Phys. B75, 605-612 (2002) and Appl. Phys. B76, 325-328 (2003), there is a case where a noise occurs when optical signals detected by the two photodetectors are converted into electric signals, and thereby causing difficulties to measure a micro-magnetic field with high accuracy. In addition, since two photodetectors are used as detectors, the structure of a magnetic sensor is complicated and the calculation also becomes complicated.